Sol-Grid: Photochromic

The “Next” State of the Art Lens Technology: The Photochromic/ Electrochromic - Automatic and Selectable Variable Density Lens.

What is photochromism?

Photochromic and Electrochromic materials turn from transparent to opaque upon exposure to increasing levels of sunlight, or by the application of a small voltage as available on so called color changing or transitional eyewear and sunglasses.

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Photochronism or the photochromic phenomenon was discovered in the late 1880s, but it wasn’t until the 1960s that they were first commercially developed and have now been in use for over 40 years.  Photochromic lenses have been made popular by brands like “Transitions Optical” among others.

How does it work?

Photochromic lenses  s  l  o  w  l  y  darken on exposure to UV or photonic radiation. Once the radiation (light) is removed,  for example by walking indoors, the lenses gradually,  s  l  o  w  l  y  return to their transparent state.

A little light history

Photochromic lenses may be made of many different materials including glass, resin or polycarbonate or other polymers.  A glass version of this type of lenses was first developed by the Corning group in the 1960s. More recently, plastic and resin versions of these lenses have been commercialized.  The first of these was the Photolite lens sold in the early 1980s by American Optical Corporation but the first commercially successful plastic photochromic lens was introduced by Transitions Optical in 1991.

Versions of these lenses on glass substrates achieve photochromic properties by embedding microcrystalline silver halides (usually silver chloride), within the glass lens. Plastic and resin photochromic lenses rely on organic photochromic molecules such as oxazines and naphthopyrans to achieve reversible opacity. These lenses darken typically in sunlight but not under artificial light since these photochromic dyes are UV (ultraviolet) sensitive. Automobile windshields and glass windows naturally block UV light to some degree which restricts the amount the lenses will darken. Recently new dyes have been developed to allow the lenses to darken in response to visible light.

The Problem with Convention

When photochromic materials are dispersed in a glass substrate, the amount of darkening or density is dependent on the thickness of glass, which poses problems with the variable thickness of lenses in prescription glasses. In conventional plastic and resin lenses, the material is typically embedded into the surface layer of the plastic in a more uniform thickness of up to 150 μm and is therefore somewhat less problematic but still not optimum.

A study by the Institute of Ophthalmology, University College London, has suggested that even in dark conditions, photochromic lenses can absorb up to 20% of ambient light, preventing the lenses from achieving true transparency and clarity.  Temperature also has a large effect on photochromic materials performance. Because photochromic compounds fade back to their clear state by a thermal process, the higher the ambient temperature, -the less density or darkness the photochromic lenses will express.  This characteristic, known as "temperature dependency" prevents current photochromic or transitional lenses from achieving total or near total density in higher, summer temperatures.  Conversely, photochromic lenses will turn very dark in cold winter temperatures.  For this reason, photochromic lenses have been limited to eyewear where the transformation from dark to light, is restricted to a very narrow range, never getting very dark or very light.

Slow, and Not Very Steady

When exposed to UV light, a typical photochromic lens will darken or achieve greater density over a period of approximately fifteen minutes with the bulk of the transformation occurring within less than one minute, then continuing to darken slowly until reaching their darkest state.  The lenses will reverse and revert back to a clear state along a similar time frame as soon as they are removed from UV light exposure.

Applied Science

There are many types of Photochromic molecules in various classes: triarylmethanes, stilbenes, azastilbenes, nitrones, fulgides, spiropyrans, naphthopyrans, spiro-oxazines and quinones among others.  The switching speed or transition speed of these photochromic dyes is highly sensitive to the rigidity of the environment surrounding the dye. These materials, when held in a test tube in a liquid state or suspension, will exhibit very fast switching times, demonstrating that they switch more rapidly in solution or “loose” environment and more slowly in a rigid environment like being embedded in a polymer or glass lens substrate. Unfortunately it has not been possible to create a lens with a “liquid” surface.  Until now, until Sol-Grid.

The Sol-Grid Solution: Getting in the Groove

Photochromic dyes are like wild animals; they are very sensitive to climate and temperature changes and just don’t like captivity or in the case of photochromic dyes, being held tightly or encapsulated in substrate materials such as a glass and plastic lens. You can observe this phenomena by looking at a test tube filled with a photochromic dye.  When a light source , such as a pen light is pointed at it, the area is almost instantly darkened.  When the light source is removed, the dye returns to a clear state very quickly.  Sol-Grid has applied this logical solution in a new patent pending technology. 

Utilizing Sol-Grid’s patent pending technology, Sol-Grid fills the grooves or recesses in a substrate created by a nano grid structure with photochromic dye and then seals it in, holding the dyes within the grooves “loosely” as if in solution. (think thermos bottle)  Because the dyes are surrounded and protected within the walls of the nano-grooves, there can be dramatic effects on the switching speed and durability of the dyes and lenses, potentially yielding vast improvements in performance.

In addition to the enhanced transition speed it is also possible to include an insulation layer such as Aerogel, further enhancing the performance capabilities of the lens by insulating the photochromic dyes from extreme temperatures.

Sol-Grid: Electrochromic

Discovering electricity once again.  Dr. Franklin would be proud.

Photochromic materials change from transparent to a opaque when it is exposed to light and reverts back to transparent when the light is dimmed or blocked. An Electrochromic material changes color or density when a small electric charge is passed through it.

Variable Density Lenses; Dial in your color.

Sol-Grid has taken another giant leap into the future with the development of the first practical and affordable electrochromic lens. Sol-Grid photochromic lenses change density, automatically adjusting to ambient light conditions.  But what if you want more choice and control? 

By creating a simple shutter device from a sandwich of Sol-Grid nanowire grids and liquid crystal (LC) molecules, Sol-Grid is able to produce a lightning fast density /color changing lens. Sol-Grid technology simplifies and reduces the parts needed to accomplish this effect, making this the first truly practical, durable and cost effective LC variable density lens system available.  Choose how dark you want your lenses and dial it in.

The images below show a cross section of a conventional LC shutter on the left and the simplified Sol-Grid LC shutter on the right.  The difference is clear to see.

Sol-Grid: Variable Density Lens options

Sol-Grid technology encompasses many advanced features, superior performance and unique abilities not possible with today’s conventional products with faster response times and a wider range of density variability.

Another advantage of the Sol-Grid technology is the ability to conduct heat and prevent hot spots from occurring.  In traditional photochromic lenses, it is possible to get “hot spots” in areas of the lens where the substrate surface temperature is higher or lower.  This is due to the fact that different areas of the lens may absorb more or less solar energy and at different rates.  Because of current transition lenses “temperature dependency”,  hot spots or dark spots may appear and cause an uneven tint in the lens.

Sol-Grid technology addresses this potential problem naturally.  The wires in the grid are able to conduct and disperse heat away from the surface, maintaining a more even, consistent surface temperature.

Future Applications

Sol-Grid innovations have applications not currently possible. For example, a ferro–electric or electrochromic attenuator would permit very fast switching which would have a multitude of applications including, laser light battle protection for military use, 3D stereoscopic applications and automatic welding goggles to name a few.

Another variation of this technology is photo-polarization for use in high performance sunglasses and products that need to be photo-polarized, Ie: a filter that becomes polarized when exposed to light energy.  This would allow the degree of polarization to be automatically adjusted as the lighting conditions change.

Both LC and photochromic devices operate better in warm conditions and more sluggishly in the cold.  Sol-Grid technology makes it is possible to utilize the wire grids as an internal heating element to generate a small amount of heat to prevent the liquid crystals and photochromic dyes from freezing, thereby increasing their performance in cold and freezing temperatures.

Sol-Grid: The most advanced Photo and Electrochromic technology available anywhere.

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